The present invention relates to a method of generating pulse trains in general, and more particularly to a method of generating a sampling signal succession for use in control systems utilizing pulse-length modulated control signals, especially for controlling the tension or position of a continuously advancing web, and to an apparatus for performing the above method.
Methods and arrangements of the type here under consideration are already known and they are frequently used, for instance, in rotary printing machines, where it is required that a paper web advance in exact registry with the desired advancement path thereof. To accomplish this goal, it is necessary to provide control arrangements in the web advancing arrangements or cooperating with such web advancing arrangements.
In rotary printing machines equipped with rollers, like in many other application instances, the simplicity of the technical solution and the devices utilized in this solution is one of the main requirements. In the event that very simple control devices, which are also realizable by using relatively inexpensive components therein are used, then three-point control circuits (or other multi-point control circuits) are often being used. Three-point control circuits are already known (for instance, from the publication Taschenbuch Elektrotechnik in sech Banden, edited by E Phillipow, Berlin, published by VEB Verlag Technik 1979, Vol. 4, p. 451). Such control circuits usually include a three-point or three-position switch having a tripping value a, and an integral member having an adjustment speed y. Typical applications are obtained when the three-position switch, for instance, causes a motor to rotate in one or in the opposite sense, depending on the sense of the deviation or error signal, or switches the motor off. In such a case, the control circuit is fully suited to bring the value being controlled into a stable condition. However, a stable operation of such a control circuit is achievable only by selecting a relatively high tripping or actuation value a and a relatively small adjustment speed y, and this is adversely reflected in the static accuracy and the settling time.
There are also already known (for instance, from the publication Taschenbuch Elektrotechnik in sechs Banden, edited by E. Phillipow, Berlin, published by VEB Verlag Technik 1979, Vol. 4. p. 452) sampling control circuits. Such control circuit find widespread applications not only for simple tasks, but also when computers are being used. Among such known sampling control circuit are those in which the value to be controlled or the control deviation or error signal is discontinously sampled during a sampling period with a constant sampling frequency, so that the output signal of the control device is available only during a part of the sampling period. Herein, the output signal can be a binary signal having a length in time that is proportional to the analog or discrete value of the error signal, that is, this output signal is produced from the error signal value by an analog or discrete pulse length modulation. The servomotor which is connected to the output of the control device is merely switched on or off, so that it runs at a constant adjustment speed during adjustment periods of different durations. The servomotor, which acts in an integrating manner, performs in addition to its adjustment function also the function of a storage member which is necessary in sampling control circuits for the continous presentation of the adjustment signal.
Especially this solution principle finds widespread acceptance as a technically closed solution because of its simplicity and its simple automatic control adaptability in a relatively large number of applications. This solution further has the advantage of good dynamic properties in apparatus with compensation, in that the control device, in a sense, waits during the sampling intermissions to find out what effect the preceding adjustment operation has had.
In the field of use of rotary printing machines equipped with rollers, it is to be borne in mind when designing web-tension or web-position control systems that what is involved is a control stretch with a compensation of the first order, the equalization time t.sub.a of which satisfies the relationship t.sub.a .apprxeq.3.multidot.L/v, wherein L is the web displacement during the equalization process and v is the web speed, that is, the equalization time t.sub.a is dependent on the web speed v. For achieving a high control factor, the sampling operation is conducted at a sampling frequency which is dependent on the web speed, in that a pulse generator or clock which is driven from the printing machine generates a predetermined number of scanning pulses per revolution of the printing cylinder. Herein, a minimum time duration length is necessary for the scanning period, which is equal to the sum of the time intervals formed from the adjustment time needed for an adjustment step with the maximum adjustment width and the equalization time necessary for the equalization. Only when the new sampling, which corresponds to the feedback indicative of the effected adjustment result, occurs after the termination of the equalization time, can the sampling and control operation be brought exactly to a standstill already after one adjustment step.
The thus produced sampling frequency has the disadvantage that it is independent of the time duration or length of the adjustment signal. Also, it remains out of consideration that the commencement in time of the equalization time depends on the time duration of the adjustment step, as well as that no equalization occurs when an adjustment action in the event of a toleratable value of the error signal, that is, that it is not necessary to wait for the expiration of the equalization time. This disadvantage stands in the way of a further increase in the control factor.
An important drawback of the known sampling control devices resides in the fact that they react only to the amount or value of the error signal. Inasmuch as the equalization time which is dependent on the web speed becomes effective even in the event of occurrence of disturbance variable action, the effect of any disturbance variable action which is captured or detected during the sampling operation while still in formation can be counteracted only by a multi-step sampling control operation. Even this drawback is an obstruction to a further increase in the control factor.